http://dev.w3.org/csswg/css3-speech/#cue-props
# The loudness of prerecorded audio cues can be adjusted relative to
# the volume level of synthetic speech, inherited value from the
# â€˜voice-volumeâ€™ property.
Given that cues are prerecorded audio, which presumably has a volume
of its own (that can fluctuate during the audio clip), how is that
volume matched to the voice-volume?
~fantasai